A check valve, or non-return valve, lets fluid pass one way and shuts the instant it tries to come back. Follow the shutter, the seat and the spring that does it all.
A check valve — also called a non-return valve — is a simple fitting that allows fluid to flow in one direction only. It has no handle and needs no power: forward flow pushes it open, and any flow trying to come back pushes it closed. That makes it the automatic one-way gate of a piping system.
From the outside it is just a short section of pipe with a bulge in the middle. The moving parts — the shutter, its seat and the spring — are all hidden inside, and they are what decide whether fluid may pass or must be blocked.
Fluid arrives from the inlet and presses on the face of the shutter. As soon as its pressure is enough to overcome the light spring, the shutter is pushed off the seat and to the right.
With the shutter lifted, a gap opens at the seat and fluid flows past it, through the guide and on to the outlet. The spring is compressed but keeps pushing back, ready to close the valve again.
The instant the flow slows or tries to come back, pressure flips to the other side of the shutter. The spring and the back pressure press it onto its seat, and because the seat narrows, the shutter closes tighter the harder the flow pushes back.
A check valve, also called a non-return valve, is a fitting that lets fluid flow in one direction only. It opens when the flow pushes it the right way and seals automatically if the flow tries to come back.
It does not sense anything — it works purely on pressure. Forward pressure pushes the shutter off its seat; any pressure from the other side pushes the shutter back onto the seat and closes it.
The spring keeps the shutter seated when nothing is flowing, so gravity and small pressure changes cannot jiggle it open. The flow only has to overcome a light spring, so the valve still opens easily.
Most are fixed and open at whatever pressure the flow provides, but spring-loaded versions come with different cracking pressures. A stronger spring means the valve opens only at a higher pressure.
Anywhere backflow must be prevented: after pumps, in water lines to stop contamination, in compressors, and beside safety devices so fluid can only travel the intended way.